The present invention relates to a method and apparatus for supporting production risers for the development of an offshore reservoir. More particularly, the present invention relates to a tension leg platform (TLP) for supporting surface accessible wellheads.
Traditional bottom-founded platforms having a fixed or rigid tower structure have been taken to their logical depth limits in the development of offshore oil and gas reserves. Economic considerations suggest that alternatives to this traditional technology be ordinarily used in deep water. Further, even the most promising reservoirs are difficult to economically exploit in this manner at any depth greater than about 1200 feet in the Gulf of Mexico and often less in other areas.
One alternative to fixed towers is to drill from facilities provided on surface vessels and to complete the wells at the ocean floor with subsea completions. Gathering lines connect the subsea wells to facilities usually located at the surface, either in the immediate vicinity or in a remote location. However, subsea wells are relatively inaccessible at the ocean floor and this fundamental problem is exacerbated by the rigors of the maintenance-intensive subsea environment. The result is complex, costly maintenance operations.
Alternatively, deepwater wells can be provided with surface completions on specialized structures more suitable for deepwater applications. One such design is the tension leg platform. Broadly, this design concept employs a floating superstructure secured to the ocean floor through tendons or tethers which are tensioned to draw the superstructure down below its free-floating draft. Such structures can provide drilling and production facilities in deep water at costs less than those of traditional fixed platforms. Nevertheless, the high cost of the traditional practice of these structures requires a high concentration of wells in order to be economically feasible.
The cost of deepwater platforms further increases with the range of well operations to be conducted from the platform. Depending on the well operations, this can substantially increase the load on the platform, thereby requiring a substantially larger structure.
For instance, a full capability drilling rig can be deployed which will allow primary drilling from the tension leg platform ("TLP"). This requires a large structure which must support a large number of wells. Many hydrocarbon reservoirs cannot effectively utilize, and therefore justify, such a number of wells. Other reservoirs can justify the number of wells, but only if extended reach drilling techniques are used to drain relatively remote areas of the reservoir from the facilities provided on the platform. This extended reach can be accomplished with the current directional and horizontal drilling techniques, but only by substantially increasing the drilling cost for the wells so extended. Further, primary drilling operations to develop a dispersed reservoir with extended reach techniques from a central location can spread the drilling operations over many years. Subsequent well workover operations may tie the drilling rig to the platform many years thereafter even though primary drilling is complete. Both aspects represent economic inefficiencies. In the first instance, drilling such extended reach wells, one well at a time, delays production, thereby adversely affecting the rate of return of the substantial capital expenditures necessary to provide such a deepwater structure. Further, after the wells have been drilled, the rig represents a very substantial asset which cannot otherwise be efficiently used and has similarly permanently committed the prospect to the larger structure, thereby affecting the cost of the platform as well.
Alternatively, the wells can be predrilled from a drill ship or other floating facility, killed or otherwise secured, and completed from a scaled-down "completion" rig carried on a production platform such as a tension leg well platform (TLWP) installed at the site later. This reduces the load on the permanent facilities and therefore permits a somewhat smaller platform, but prevents production from any well until all the wells have been drilled and thereby substantially defers revenue from the development. Further, this scheme does not allow the flexibility to permit additional or replacement drilling once the platform has been installed.
Efficient development of deepwater hydrocarbon reserves must overcome these deficiencies and provide for developing the reservoirs with lower capital outlays, faster return on investment, more efficient reservoir management for larger reservoirs, and enhanced profitability for reservoirs that are otherwise marginal. The present application is for a platform design which facilitates methods disclosed in copending applications which, together, provide the benefits referenced above.